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Fuel Cap Pressure Release?

86M10086.2L

Member
387
2
18
Location
Long Island, New York
I walked out to my truck about a week ago and noticed a small drip of fuel under the truck. It had been comming from the fuel filler hose where it mates to the tank. I tightened up the clamp, and made the assumption that I just overfilled the tank and ther leak hasn't come back. What I have been noticing though. Everytime I take the fuel cap off, I'm getting a pressure release from the tank like when you open a soda/beer bottle. Is that normal? I never really noticed it before but then again I was never really looking. Truck runs fine. Just one of those little things I wasn't too sure about.
 

maritimer

New member
404
3
0
Location
Yarmouth, NS
mine does it too, never had a problem with ti so never tried to figure it out lol, but i am interested t oknow what it is and if ti is a problem
 

maybefixit

New member
106
1
0
Location
Hamilton, Ohio
Most fuel caps are set up to hold air pressure in the system, and allow air in when there is reduced pressure in the tank. This is to prevent fuel vapors from escaping the fuel system and getting into the atmosphere. Generally there will be a method for getting the fuel vapors to the engine. In gasoline engined cars, this is a charcoal canister which has a vacuum hose going to the intake manifold. There is another line going to the tank with a check valve, so that vapors stay in the tank unless the engine is running to draw the fumes into the engine and burn them.

This time of year, it's not unusual to see air pressure build up when you have a cold overnight, making the fuel system 'breathe in' through the cap, but can't 'breathe out' the next (warmer) day due to its design. When you remove the cap it lets out that warmed-up air that's built up some pressure.

The problem generally will be worse when the tank is near empty, since a cold tank overnight will take in more air which will expand over the day. A tank that's mostly full won't have room for air, and the fuel won't expand or evaporate much to cause the pressure to build up.

With the CUCV's being mostly 'off the shelf' GM trucks, I'd expect that their fuel caps are the same as the diesel caps for civi vehicles, so they'd be designed to meet the Clean Air Act specification. I'm not sure about the rest of the fuel system, since I don't have a CUCV myself to compare with.
 

86M10086.2L

Member
387
2
18
Location
Long Island, New York
The plot thickens..... I come home from work today and again I see a little wet spot underneath the truck by the fuel filler. I open up the fuel cap get the same pressure release like before but I observe the fuel recirculating from overflow tube back into the filler neck. Am I missing something here? Or am I just over filling the tank? I've filled the tank far more in the past and never had a problem. Is there some type of venting sytem thats clogged? I know diesel expands but I've had this truck for a year and a half and it's never done this before, and it's only 40 degrees out. Thoughts? ideas? Suggestions of which hand to scratch my head with as I stare at the problem?
 
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